(1) Field of the Invention
The present invention relates to an aqueous dispersion of a hydrophobic resin capable of forming a resin film excellent in water resistance, oil resistance, chemical resistance and adhesion to a hydrophobic material, and a process for the preparation of this aqueous dispersion.
More particularly, the present invention relates to an aqueous dispersion of a hydrophobic resin, in which the hydrophobic resin is maintained in the form of oil-in-water type dispersed particles even if the water content is very low. Furthermore, the present invention relates to a process for preparing an aqueous dispersion, according to which not only the above-mentioned aqueous dispersion but also an aqueous dispersion having a high water content can be freely prepared.
(2) Description of the Prior Art
Various aqueous dispersions of polymers are known. For example, a flowable aqueous dispersion having a water content higher than about 30% by weight (hereinafter referred to as "liquid dispersion") is coated and dried on the surface of a paper or fibrous article, a plastic molded article, a wood article or a metal to form a resin film imparting water resistance, oil resistance and chemical resistance to the substrate or is used as a heat-sealing agent. Since water is used as a dispersion medium in this aqueous dispersion, the dispersion is advantageous over solvent type dispersions because there is no problem of ignition or environmental pollution and the dispersion can be easily handled. Accordingly, aqueous dispersions of this type are widely used in various fields.
As another type, there is known an aqueous dispersion which has no flowability and is seemingly solid (hereinafter referred to as "solid dispersion"). Namely, this emulsion is powdery and called "powder emulsion", and when water is added to this solid emulsion, re-dispersion is caused to form a liquid aqueous dispersion. This solid dispersion does not contain water at all, or if it contains water, the water content is 2 to 3% by weight at highest. Accordingly, there is no risk of freezing at a low temperature, packaging or transportation is simplified, and a narrow space is sufficient for the storage. Moreover, the solid dispersion can be mixed directly with a powder or granules for which the contact with water is not desired, such as cement, mortar or gypsum, and a composition having a high processability can be prepared.
Known solid dispersions cannot be prepared by the same process as adopted for the preparation of liquid aqueous dispersions. Namely, this solid dispersion is prepared by once forming a liquid aqueous dispersion having a higher water content according to various known processes and spraying the liquid aqueous dispersion in hot air in a furnace by a sprayer to evaporate water and recover a powder. This process, however, is disadvantageous in that when a liquid aqueous dispersion of a polymer having a low softening temperature is used as the starting material, agglomeration of polymer particles is caused at the spraying step or the formed powdery emulsion is aggregated into a bulk under the action of heat or pressure. Even if water is added to this powder emulsion to effect re-dispersion, dispersion is not smoothly effected or even if dispersion is effected, the viscosity is increased to degrade physical properties of the formed coating film. In order to eliminate this disadvantage, an additive such as an anti-sticky agent or a protecting colloid is added to a liquid aqueous dispersion before spraying or to a powder before drying, but if the additive is not incorporated in an extremely large amount, no satisfactory effect is obtained, and if the amount of the additive is increased, the physical properties of the formed coating film are degraded. Moreover, since water contained at a high content is evaporated, energy loss is great and the process is economically disadvantageous. Therefore, development of a technique of preparing a solid dispersion such as a powder emulsion while reducing the amount of an additive to a level as low as possbile by a process having reduced energy loss has been eagerly desired.
Moreover, improvements are desired also in the process for the production of liquid aqueous dispersions. The known process is roughly divided into a process in which emulsion polymerization is carried out in an aqueous medium in the presence of an emulsifier and a process in which a molten resin is stirred and mixed with an aqueous medium under a shearing force. The former process is defective in that the kinds of resins to be formed by polymerization are limited, and control of the polymerization reaction is complicated and a complicated apparatus has to be used. On the other hand, the latter process is advantageous in that the process can be applied to any resin and the operation and apparatus are relatively simple.
Various proposals have been made on the latter process. For example, Japanese Patent Application Laid-Open Specification No. 12835/76 discloses a process in which a kneaded mixture of a thermoplastic resin and a water-soluble polymer is dispersed in water. However, since a film formed from an aqueous dispersion prepared according to this process contains a water-soluble polymer, the film is poor in the mechanical strength or water resistance. Japanese Patent Publication No. 23703/82 discloses a process in which a kneaded mixture of polypropylene and a dispersant selected from a surfactant and a water-soluble or water-swelling polymer is dispersed in water. The particle size of dispersed particles is relatively large, and in order to reduce the particle size, it is necessary to use a petroleum resin in combination. Accordingly, a film obtained from an aqueous dispersion prepared according to this process is sticky. Japanese Patent Application Laid-Open Specification No. 2149/81 discloses a process in which an aqueous dispersion is prepared by dispersing an olefin resin with an aqueous solution of partially saponified polyvinyl alcohol. Since the dispersion contains a water-soluble resin, the physical properties of a film formed from the dispersion are poor. Moreover, the dispersion obtained according to this process is a liquid dispersion having a water content higher than 30% by weight. Moreover, Japanese Patent Publication No. 42207/83 discloses a technique of forming a dispersion by melt-kneading a polyolefin with a carboxyl group-containing polyolefin, feeding the mixture in hot water containing a basic substance and applying a shearing force to the mixture. This process is advantageous in that the obtained dispersion does not contain a water-soluble or water-swelling polymer or a petroleum resin is not used. However, if it is intended to prepare a product having a small dispersed particle size, the kinds of starting resins are limited. Moreover, since the dispersing step is carried out at a high temperature under a high pressure, a vessel having a pressure resistance should be used for the dispersing operation. In the foregoing processes proposed for preparing aqueous dispersions, development of means of forming an aqueous dispersion of fine particles without using a water-soluble or water-swelling polymer or a petroleum resin, which can be applied to all the resins, is eagerly desired.
Furthermore, development of a process in which a liquid aqueous dispersion and a solid dispersion can be optionally and freely prepared without adopting any special step is desired.